1. Field of the Invention
The invention relates generally to optical grating devices, and more particularly to techniques for athermalizing such devices.
2. Description of Related Art
Computer and communication systems place an ever-increasing demand upon communication link bandwidths. It is generally known that optical fibers offer a much higher bandwidth than conventional coaxial links. Further, a single optical channel in a fiber waveguide uses a small fraction of the available bandwidth of the fiber. In wavelength division multiplexed (WDM) optical communication systems, multiple optical wavelength carriers transmit independent communication channels along a single optical fiber. By transmitting several channels at different wavelengths into one fiber, the bandwidth capability of an optical fiber is efficiently utilized.
Fiber-optic multiplexing and demultiplexing have been accomplished using an arrayed waveguide grating (AWG) device. An AWG is a planar structure comprising an array of waveguides disposed between input and output couplers and arranged side-by-side with each other, and which together act like a diffraction grating in a spectrometer. Each of the waveguides differs in length with respect to its nearest neighbor by a predetermined fixed amount. The outputs of the output coupler form the outputs of the multiplexing and demultiplexing device. In operation, when a plurality of separate and distinct wavelengths are applied to separate and distinct input ports of the device, they are combined and are transmitted to an output port. The same device may also perform a demultiplexing function in which a plurality of input wavelengths on one input port of the apparatus, are separated from each other and directed to predetermined different ones of the output ports. AWGs can also perform a routing function, in which signals arrive on multiple input ports and are routed to multiple different output ports in accordance with a predefined mapping. The construction and operation of such AWGs is well known in the art. See for example, “PHASAR-based WDM-Devices: Principles, Design and Applications”, M K Smit, IEEE Journal of Selected Topics in Quantum Electronics Vol. 2, No. 2, June 1996, and U.S. Pat. No. 5,002,350 and WO97/23969, all incorporated by reference herein.
Wavelength division multiplexers and demultiplexers require precise control of the effective optical path length difference between adjacent waveguides. The effective optical path length difference is defined as the product of the effective index of refraction of the fundamental mode in the waveguide and the physical path length difference between adjacent waveguides. The effective index of refraction of the fundamental mode in the waveguides and the physical path length differences between adjacent waveguides for currently available wavelength division multiplexers and demultiplexers are typically both temperature dependent. In conventional integrated optical multiplexer and demultiplexer devices, the medium forming the arrayed waveguides has a noticeable temperature dependency which results in changes in the central transmission wavelength which may exceed the transmission bandwidth. As a result, temperature variations that are within a specified device operating temperature range (e.g. from about 0 C to about 70 C) induce a wavelength shift which is unacceptable in comparison to the typical accuracy requirements. Consequently, available multiplexer/demultiplexer optical devices of the phased array type are generally operated in a temperature controlled environment. Typically, control circuits with heating elements are provided to maintain the device at a stable temperature higher than the maximum specified operating temperature. But the use of heating elements to achieve active athermalization is undesirable because it increases the overall cost, size and complexity of the device, reduces device lifetimes, and consumes considerable power. It also usually requires active smart control electronics and even then it may operate differently depending on the device's physical horizontal/vertical orientation. Peltier coolers can also be used, but these suffer from many of the same inadequacies.
In the case of conventional wavelength division multiplexers having a phased array optical grating comprising a plurality of silica waveguides and silica cladding, the variation of channel wavelength as a function of temperature predominately depends on the positive variation of the effective index of refraction of the waveguides as a function of temperature. In an effort to compensate for the positive variation of refractive index as a function of temperature for silica-based materials, polymer overcladding materials having a negative variation of refractive index as a function of temperature have been employed. However, a problem with this arrangement is that as the temperature varies, the difference in refractive index between the core and the cladding varies, and in the worst case, light may not be able to be guided into the waveguide. As a result, optical multiplexer/demultiplexer devices having a phased array type grating with a polymer overcladding may not be suitable for use over a wide range of ambient temperatures.
Another proposed design for maintaining a relatively constant effective optical path length difference between adjacent waveguides in a phased array involves localizing a polymer in a triangular or crescent-shaped groove either in the phased array or in the slab region coupling the phased array with either the input or output fibers. The polymer can be selected such that it has a negative variation in effective index of refraction as a function of temperature to compensate for the positive variation in the index of refraction of the silica waveguide core segments as a function of temperature, thereby inhibiting shifting of channel wavelengths due to variations in operating temperature within a predetermined operating temperature range. The polymer groove can be divided into more than one groove encountered by the optical energy sequentially, to reduce the length of free space propagation across each groove.
The use of polymer-filled grooves can improve athermalization substantially. Typical AWGs that have been athermalized in this way can achieve a center channel wavelength drift as small as 0.03 to 0.05 nm over a typical operating temperature range of −5 to +70 C. However, that is still not good enough. Such drifts limit the applicability of the device to only that stated temperature range, and to only systems having channel spacings of about 100 GHz or higher, where this variation would be tolerable. They are not readily usable, for example, in an outdoor equipment enclosure in climates where freezing temperatures are possible, or in systems that require a broad passband and a channel spacing less than about 100 GHz.
Another major category of techniques that have been investigated for athermalization are mechanical in nature, such as techniques that include temperature-controlled actuators for actively positioning the components of the device relative to each other. These may include, for example, a bimetallic actuator that adjusts the lateral position of the input waveguide relative to the input slab region in accordance with ambient temperature. These techniques are generally complex and expensive to make as the manufacturing tolerances are usually extremely tight.
Accordingly, there is an urgent need for arrayed waveguide grating devices that exhibit much better athermalization over a wider temperature range than has previously been possible or practical, without requiring a temperature controlled environment, and without requiring the complexities and tight manufacturing tolerances of mechanical methods.
In existing polymer-filled groove athermalization methods, the change in refractive index of both the silica based waveguide material and the polymer compensation material, are both assumed to be linear with temperature. Any higher order effects typically are ignored. Most references that characterize the refractive index change of a material with respect to temperature, also state only a linear relationship between the two when measured at temperatures away from the glass transition temperature of the polymer. Applicants have recognized that the relationship is usually not exactly linear, and that the deviation of these variations from the linear may be responsible for a significant part of the imperfect athermalization observed in these devices. In an aspect of the invention, therefore, roughly stated, the selection of materials takes into account at least the second order effect of the materials. As a result, a polymer compensation material can be identified that compensates for the effective optical path length variation in the waveguide material with much better accuracy, or over a much wider temperature range of −30 C to +70 C for example, or both.
In another aspect of the invention, again roughly described, two different compensation materials are used for filling a plurality of slots or compensation regions inserted in the optical paths. The optical path length vs. temperature curves of the two compensation materials are characterized to at least the second order, as is that of the base waveguide material. The two compensation materials are placed in different numbers of the grooves in an appropriate ratio, so as to create the ratio of effective interaction lengths that is required to accurately minimize the temperature dependence of the overall optical path length to both the first and second order. The technique is generalizeable to any number of different compensation materials, and to neutralization of the optical path length temperature dependence to any order of characterization.
In one aspect of the invention, roughly described, optical apparatus has a plurality of passbands and a center wavelength, and the first through Q'th order derivatives with respect to temperature of the center wavelength, Q>=2, are substantially equal to zero throughout a temperature range of 0 C to +70 C, −5 C to +70 C, −30 C to +70 C, or −50 C to +90 C. The apparatus can include a plurality of optical paths carrying optical energy from the input port to the output port through a plurality of materials, each of the materials having an effective index of refraction temperature dependency which differs from that of the other materials. Alternatively or additionally, the apparatus can include a waveguide in optical communication with a particular one of the input and output ports, and a temperature compensation member that adjusts the physical position of the waveguide with respect to the arrayed waveguide grating in dependence upon temperature.
In another aspect of the invention, roughly described, optical apparatus has comprising a plurality of optical paths through a material system, each of the optical paths having a respective effective optical path length which differs from that of an adjacent optical path by a respective effective optical path length difference, and the first through Q'th order derivatives with respect to temperature of each of the optical path length differences, Q>=2, are substantially equal to zero throughout a temperature range of 0 C to +70 C, −5 C to +70 C, −30 C to +70 C, or −50 C to +90 C. The apparatus can include a plurality of optical paths carrying optical energy from the input port to the output port through a plurality of materials, each of the materials having an effective index of refraction temperature dependency which differs from that of the other materials. Additionally, each x'th one of the materials can have a respective total physical propagation distance along each of the optical paths which differs from the total physical propagation distance through the x'th material along an adjacent one of the optical paths by a respective physical path length difference ΔLx, each of the ΔLx's remaining substantially constant with temperature throughout the temperature range.
In another aspect of the invention, roughly described, optical apparatus has a plurality of optical paths through a material system, each of the optical paths traversing at least three materials, each of the materials having an effective index of refraction temperature dependency which differs from that of the other materials, each of the optical paths having a respective effective optical path length which differs from that of an adjacent optical path by a respective effective optical path length difference, and the first through Q'th order derivatives with respect to temperature of each of the optical path length differences, Q>=1, are substantially equal to zero throughout a temperature range of 0 C to +70 C, −5 C to +70 C, −30 C to +70 C, or −50 C to +90 C. In an embodiment, each x'th one of the materials has a respective total physical propagation distance along each of the optical paths which differs from the total physical propagation distance through the x'th material along an adjacent one of the optical paths by a respective physical path length difference ΔLx, each of the ΔLx's remaining substantially constant with temperature throughout the temperature range.
In another aspect of the invention, roughly described, an arrayed waveguide grating apparatus has a plurality of optical paths from an input to an output, comprising a base material and at least one compensation region, the at least one compensation region collectively containing at least first and second compensation materials intersecting the optical paths and having effective index of refraction temperature dependencies that differ from each other and from that of the base material. In one embodiment, a first one of the compensation regions includes both the first and second compensation materials. The first and second compensation materials can be disposed in different layers in the first compensation region. Alternatively, the first compensation region can further include a third compensation material, where the first compensation material is disposed in a lower layer in the first compensation region, the second compensation material is disposed in a middle layer in the first compensation region, and the third compensation material is disposed in an upper layer in the first compensation region, and where the second compensation material has an index of refraction higher than both that of the first and third compensation materials. Where the arrayed waveguide grating apparatus includes in the base material a lower cladding layer, a core layer superposing the lower cladding region and an upper cladding layer superposing the core layer, and the second compensation material can be is substantially coplanar with the core layer in the base material.
In another embodiment, roughly described, one of the compensation regions can include the first compensation material and not the second compensation material. In such an embodiment the at least one compensation region can collectively contain a plurality of compensation materials including the first and second compensation materials, all of the compensation materials in the plurality of compensation materials intersecting the optical paths and having effective index of refraction temperature dependencies that differ from each other and from that of the base material, and wherein each of the compensation regions includes exactly one of the compensation materials.
In either of the above embodiments, roughly described, the first compensation material can include a composite plurality of sub-materials, the effective index of refraction temperature dependence of the composite being the effective index of refraction of the first compensation material. The sub-materials can be layered to form the composite. Alternatively, a first one of the sub-materials can be the same as the base material and a second one of the sub-materials has an effective index of refraction temperature dependence that differs from that of the base material and from that of the composite.
In an embodiment, one of the compensation materials can further compensate for bi-refringence of the base material.
In another aspect of the invention, roughly described, an arrayed waveguide grating apparatus has a plurality of optical paths from an input to an output, including a base material and a plurality of compensation regions, a first subset of at least one of the compensation regions containing a first compensation material and a second subset of at least one of the slots containing a second compensation material, wherein the first and second compensation materials have effective index of refraction temperature dependencies that differ from each other and from that of the base material. In an embodiment, the base material might comprise a silica and the first and second compensation materials might be polymers.
In another aspect of the invention, roughly described, optical apparatus has a plurality of optical paths through a material system, each of the optical paths traversing a number X materials, and wherein
      (                                        n                          1              ,              0                                                            n                          1              ,              1                                                ⋯                                      n                          1              ,                              X                -                1                                                                                      n                          2              ,              0                                                            n                          2              ,              1                                                ⋯                                      n                          2              ,                              X                -                1                                                                          ⋮                          ⋮                                                                          ⋮                                                  n                          Q              ,              0                                                            n                          Q              ,              1                                                ⋯                                      n                          Q              ,                              X                -                1                                                          )    ·      (                                        Δ            ⁢                                                  ⁢                          L              0                                                                        Δ            ⁢                                                  ⁢                          L              1                                                            ⋮                                                  Δ            ⁢                                                  ⁢                          L                              X                -                1                                                          )  is substantially equal to zero, where each nq,x is a q'th derivative with respect to temperature of the effective index of refraction of each x'th one of the materials, where each ΔLx is the total physical path length increment of the material x between adjacent optical paths, and wherein Q>=2 or X>=3 or both. In an embodiment, X>=Q+1. Where the X materials consist of a base material and X−1 compensation materials, the apparatus can include at least X−1 compensation regions formed in the base material, each of the compensation regions containing exactly one of the compensation materials, the compensation regions being allocated to the X−1 compensation materials substantially in proportion to the total physical path length increments ΔLx of the compensation materials.
In another aspect of the invention, roughly described, optical apparatus has a plurality of optical paths through a base material having a first trench containing a different material intersecting the optical paths, and at least one of the upstream and downstream edges of the first trench is slanted relative to the vertical by a slant angle between 5 and 20 degrees. The base material may further have a second trench intersecting the optical paths, wherein both the upstream and downstream edges of both the first and second trenches are slanted relative to the vertical by a slant angle between 5 and 20 degrees.
In another aspect of the invention, roughly described, optical apparatus has a plurality of optical paths through a base material having a plurality of compensation regions formed therein, the compensation regions containing compensation material for compensating a thermal dependency of the base material effective index of refraction, and wherein the compensation regions are disposed consecutively along the optical paths on a pitch that varies linearly from the most upstream one of the compensation regions to the most downstream one of the compensation regions. In an embodiment, wherein a particular one of the optical paths carries optical energy having a particular wavelength through the compensation regions, the pitch variation from the most upstream one of the compensation regions to the most downstream one of the compensation regions is approximately equal to M times the particular wavelength, where M is an integer and is preferably equal to 1.
In another aspect of the invention, roughly described, optical apparatus has an arrayed waveguide grating device having an output port in optical communication with an input port via an arrayed waveguide grating, a waveguide in optical communication with a particular one of the input and output ports, and a temperature compensation member that adjusts the physical position of the waveguide laterally with respect to the arrayed waveguide grating substantially in accordance with the function
      y    =                  ∑                  q          =          0                Q            ⁢                          ⁢                        k          q                ⁢                  T          q                      ,          ⁢      Q    >=    2    ,for predetermined values of each of the kq's and throughout a temperature range of −5 C to +70 C. In an embodiment, in which Q=2, and in which the arrayed waveguide grating device has an index of refraction approximated throughout the temperature byn=n0+n1T+n2T2,k1 may be related to k2 substantially by:
            k      1              k      2        =                    n                  1          ⁢          g                            n                  2          ⁢          g                      .  
In another aspect of the invention, roughly described, optical apparatus has an arrayed waveguide grating device, including an output port in optical communication with an input port via an arrayed waveguide grating, wherein the apparatus has a plurality of passbands including a subject passband, the subject passband having a center wavelength, and wherein the center wavelength varies by less than 70 pm over temperature range −50 C to +90 C. In an embodiment, the center wavelength varies by no more than 40 pm over temperature range −50 C to +90 C. Alternatively, the center wavelength of the subject passband varies by less than 20 pm over temperature range 0 C to +70 C. In an embodiment, the center wavelength varies by no more than 10 pm over temperature range 0 C to +70 C.